1. Field of the Invention
This invention relates generally to filtration cassettes, and, more particularly to methods of fabricating cross-flow filtration cassettes. Cassettes of the invention are characterized, in part, by a series of sealing channels which selectively seal the filtrate and retentate ports within the cassettes and prevent undesired flow. Methods of the invention produce filtration cassettes with an increased active filter surface area. Additionally, the present invention allows for the fabrication of filtration cassettes with a minimum of labor and at a reduced cost.
2. Background
Filtration cassettes are used in a variety of biotechnology and food processing applications. These cassettes typically comprise a stacked assembly of porous membrane components and filtrate and retentate flow screen components. During cross-flow filtration, a suspension is continually pumped into the retentate ports and through the cassette. Product that passes through the membrane is collected and passed out, e.g., drained, through the filtrate ports.
Filtration cassettes have conventionally been made using a multi-step process (see, e.g., U.S. Pat. No. 4,715,955). Generally, unused manifold ports on each of the individual flow screens (e.g., filtrate and retentate) are sealed. For example, in the case of the filtrate flow screen, ports utilized for the distribution of retentate on other layers must be sealed about their perimeter to prevent mixing. This step can also be accomplished while binding one or more flow screens to the filtration membrane layer in the production of a subassembly. By either mode, a fixture is required to hold the flow screen in place and introduce resin and vacuum appropriately.
Next, component membranes or subassemblies and flow screens which have been sealed as described above are layered within a mold or other fixture according to the design requirements for the finished filtration cassette. Once the components have been assembled within the mold, a resin is introduced about the periphery. Using conventional vacuum transfer molding techniques, the resin is drawn into the edges of the cassette assembly. Once hardened, the resin provides structural support and edge sealing around the cassette.
The resulting fabricated filtration cassette may then be utilized for its intended application by fixing it between end plates, which provide appropriate manifolding and a means of compression. The compression allows sealing between the filtration cassette and the manifold within the endplate, as well as sealing between the components of the cassette itself. This conventional fabrication process is time intensive, costly and cannot be easily automated. Further, the compression required for sealing of the fabricated cassette, both between components and between the cassette and the manifolds within the end plate, results in reduced seal reliability.
World Publication WO 03/080226, the entirety of which is hereby incorporated by reference, details our own advancements in the field. In that publication, we describe a novel filtration cassette and related manufacturing method which offer, among other benefits, a substantial reduction in time and cost of assembly. Cassettes reported therein are characterized, in part, by a series of sealing channels which selectively seal the filtrate and retentate ports and prevent undesired flow. That feature eliminates the need for a separate step to seal individual components prior to assembly of the filtration cassette.
Despite even our own advancements in the field, there remains a need for other improved cassette designs. For instance, it would be highly desirable to develop filtration cassettes which offer an increased active filter surface area (e.g., having a reduced surface area devoted to sealing channels, manifolding and/or other structures). It also would be highly desirable to develop new filtration cassettes and methods which produce repeatable sealing geometry without requiring a precise vacuum (or pressure).